1. Field of the Invention
This invention relates to the conversion of chemical energy to electrical energy. More particularly, this invention relates to the preparation of an improved cathode active material for non-aqueous lithium electrochemical cells, and still more particularly, a reaction product of xcex3-phase silver vanadium oxide (Ag0.8V2O5.4), xcex5-phase silver vanadium oxide (SVO, Ag2V4O11) and silver metal. This active product is the result of a synthesis technique including a silver-containing material and a vanadium-containing material having a silver to vanadium mole ratio of 1:2 and reacted under an atmosphere of reduced oxygen. The resulting mixed phase active material incorporated into a lithium cell provides a cathode electrode of decreased resistance and, hence, improved rate capability in comparison to cathodes of a single phase SVO material. The product cathode active material is useful in an implantable electrochemical cell, for example of the type powering a cardiac defibrillator, where the cell may run under a light load for significant periods interrupted from time to time by high rate pulse discharge.
2. Prior Art
Silver vanadium oxide (SVO) is normally prepared by heating appropriate amounts of a silver-containing compound with a vanadium oxide under static conditions in the presence of air. Such a decomposition reaction is described in U.S. Pat. Nos. 4,310,609 and 4,391,729, both to Liang et al., which are assigned to the assignee of the present invention and incorporated herein by reference. The decomposition reaction occurs under an air atmosphere at a temperature of about 360xc2x0 C. Specifically, Liang et al. discloses the preparation of silver vanadium oxide by a thermal decomposition reaction of silver nitrate with vanadium oxide conducted under an air atmosphere to produce xcex5-phase silver vanadium oxide having the formula Ag2V4O11.
A decomposition reaction is further detailed in the publication: Leising, R. A.; Takeuchi, E. S. Chem. Mater. 1993, 5, 738-742, where the synthesis of SVO from silver nitrate and vanadium oxide under an air atmosphere is presented as a function of temperature. In another reference: Leising, R. A.; Takeuchi, E. S. Chem. Mater. 1994, 6, 489-495, the synthesis of SVO from different silver precursor materials (silver nitrate, silver nitrite, silver oxide, silver vanadate, and silver carbonate) is described. The product active materials of this latter publication are consistent with the formation of a mixture of SVO phases prepared under argon, which is not solely xcex5-phase Ag2V4O11.
Also, the preparation of SVO from silver oxide and vanadium oxide is well documented in the literature. In the publications: Fleury, P.; Kohlmuller, R. C. R. Acad. Sci. Paris 1966, 262C, 475-477, and Casalot, A.; Pouchard, M. Bull Soc. Chim. Fr. 1967, 3817-3820, the reaction of silver oxide with vanadium oxide is described. Wenda, E. J. Thermal Anal. 1985, 30, 89-887, present the phase diagram of the V2O5-Ag2O system in which the starting materials are heated under oxygen to form SVO, among other materials. Thus, Fleury and Kohlmuller teach that the heat treatment of silver- and vanadium-containing starting materials under a non-oxidizing atmosphere (such as argon) results in the formation of SVO with a reduced silver content.
The prior art further describes conducting a decomposition reaction with a lower percentage of a silver-containing compound in the presence of vanadium pentoxide, resulting in the formation of a silver deficient xcex3-phase silver vanadium oxide (Ag0.74V2O5.37) along with xcex2-phase SVO (Ag0.35V2O5.18) This is described in U.S. Pat. No. 5,545,497 to Takeuchi et al. In addition, U.S. Pat. No. 6,171,729 to Gan et al. shows exemplary alkali metal/solid cathode electrochemical cells in which the cathode may be an SVO of xcex2-, xcex3- or xcex5-phase materials.
It should be pointed out that various references list xcex3-phase SVO as Ag0.74V2O5.37 or Ag0.8V2O5.4, however, they are essentially the same. For example, V. L. Volkov, A. A. Fotiev, N. G. Sharova, L. L. Surat, Russ. J. Ignore. Chem. 21 (1976) 1566 list xcex3-phase SVO as Ag0.74V2O5.37. Other references list this material as Ag0.8V2O5.4 and the two formulations for xcex3-phase SVO are equivalents for the purpose of this invention.
However, none of the prior art methods is capable of producing a low resistance cathode material as a combination of xcex3-phase silver vanadium oxide (Ag0.8V2O5.4), xcex5-phase silver vanadium oxide (SVO, Ag2V4O11) and silver metal, as per the current invention. Therefore, based on the prior art, there is a need to develop a process for the synthesis of a mixed phase metal oxide including silver vanadium oxide and silver metal and having a relatively low resistance. The product is a cathode active material useful for non-aqueous electrochemical cells having enhanced discharge characteristics, including the high pulse capability necessary for use with cardiac defibrillators.
The current invention relates to the preparation of an improved cathode active material for non-aqueous lithium electrochemical cells, and in particular, one containing xcex3-phase SVO and xcex5-phase SVO as well as elemental silver. The reaction product possess a relatively resistance (Rdc) in comparison to SVO prepared by prior art techniques. The present synthesis technique is not, however, limited to silver salts since salts of copper, magnesium and manganese can be used to produce relatively low resistance active materials as well.
These and other aspects of the present invention will become increasingly more apparent to those of ordinary skill in the art by reference to the following description taken in conjunction with the accompanying drawings.